The present invention relates to a wet stirred ball mill which effects grinding of a slurry raw material by stirring it together with grinding media such as steel balls, ceramic balls, glass beads, etc. into fine particles in the mill body, particularly to a wet stirred ball mill, wherein a slurry raw material is fed into the mill body of the closed type mill and is continuously exhausted as it is milled with the grinding medium under pressure.
This type of wet stirred ball mill is well known and available in many variations. The most popular one is of a type wherein a slurry raw material is fed from the bottom of the mill body, milled with a grinding medium by stirring under pressure, from which the medium is then separated, to exhaust the material by allowing it to overflow together with the waste media from the top of the mill body.
Now referring to the feeding mode, while there is Attritor mill Model S, manufactured by Union Process Inc. of Akron, Ohio, in which a slurry raw material is fed from the top of the mill body and exhausted from the bottom thereof, this type of mill actually is that of the open top type and resort is made to batchwise operation in which stirring is conducted under application of no pressure and it is not a pressurizing continuous mill.
In either type of the mills described above, the resulting slurry product to be exhausted from the mill must be separated from the medium. For such a purpose, a screen is often provided at the exhaust port through which the slurry product is allowed to be exhausted. However, plugging is liable to occur particularly in the pressurizing mill. A method of separating the medium other than using a screen is also known, in which a rotor fitted in the exhaust port is adapted to be driven for rotation to allow the slurry product to be exhausted through the annular clearance formed around the rotor and to separate, on the clearance, the grinding medium having a larger size than the aperture of the clearance. In such a construction, while plugging in the clearance may not easily be caused, the peripheral edge of the exhaust port and the rotor may easily be subject to abrasion. Thus, even the medium which is still usable may also be exhausted easily. Moreover, an inclusion of the medium in the clearance may be caused during the rotation of the rotor, and if it should happen and if the medium is made of ceramic, said medium will be broken by said inclusion.
Also, there has been proposed a device in which the exhaust port is adapted to be vibrated for preventing plugging thereof. The one disclosed in Japanese Provisional Patent Publication No. 216747/1986 may be given as an example of such type of mill, in which a conical (cione-shaped) valve is fitted at the exhaust port with sufficient clearance and designed to be vibrated to prevent plugging of the exhaust port around said valve. However, in this device the grinding medium is exhausted together with the slurry product continuously, and the medium is separated outside the mill body. Accordingly, the valve does not have a function to separate the grinding medium. It is possible to impart a function of separating the grinding medium to the valve by narrowing the clearance of the exhaust port. In such an instance, however, it is not possible to eliminate the plugging by enlarging the clearance only when plugging has occurred, since the valve vibrates irrespective of the presence or absence of plugging around the valve, to cause increase and decrease of the clearance repeatedly. Moreover, in such a mill, the gain and loss of the clearance caused by the vibration is constant, thereby plugging may not be eliminated in some cases depending on the degree of plugging. Although it is possible to overcome the problem of plugging by making the gain and loss of clearance larger, a wider clearance will be provided, when it is not desired, to increase the amount of exhaust. Accordingly, it becomes difficult to keep the liquid level constant, and also the dwell time that the slurry raw material dwells in the mill body will be reduced, making it impossible to obtain a product with a desired particle size through one pass. Moreover, the medium which is still effective will be easily be exhausted.